1. Field of the Invention
The present invention relates to a phosphor-free light-emitting heterojunction nitride/zinc oxide based compound semiconductor device. More particularly, it relates to a light-emitting compound semiconductor device implemented by energy band gap engineering of nitride and zinc oxide semiconductor materials to emit longer wavelengths ranging from green to orange to red (a wavelength range of 500-700 nanometers) by changing the composition of a compound semiconductor constituting an active layer.
2. Description of the Related Art
Conventional phosphors used in fluorescent lighting are not ideal for solid-state lighting because they have poor absorption and conversion efficiency. Recently, indium nitride (InN) material has been reported to have a fundamental energy band gap of approximately 0.7 electron-Volts (eV) [V. YU. DAVYDOV et al., “Absorption and Emission of Hexagonal InN. Evidence of Narrow Fundamental Band Gap,” Phys. Stat. Solidi, Feb. 1, 2002, (b) 229, R1-R3, Wiley-VCH Verlag Berlin GmbH, 13086 Berlin 2002], which is dramatically smaller than a previously reported band gap of about 1.9 eV for this material. This recent report of a relatively small band gap in InN can offer new opportunities for indium gallium nitride (InGaN) alloys to implement long wavelength light emitters, including yellow and orange/red spectral region. However, these long wavelength LEDs based on nitride quantum well active layers experience higher dislocation density, increased point defects, and severe indium (In) phase separation. Therefore, low internal quantum efficiency is expected for long wavelength LEDs based on nitride materials. Therefore, current technology to produce emitters utilizes conversion of blue or UV light emitters by encapsulating one or two different phosphors to create lower energy photon emission.
The uniqueness of zinc oxide (ZnO) over III-V semiconductor materials including large exciton binding energy [D. M. BAGNALL et al., “High temperature excitonic stimulated emission from ZnO epitaxial layers,” Applied Physics Letters, Aug. 24, 1998, Vol. 73, No. 8, pp. 1038-1040, American Institute of Physics, College Park, Md.] and high resistance to radiation damage has attracted much interest to develop high-efficient optical applications such as low threshold UV lasers [Z. K. TANG et al., “Room-temperature ultraviolet laser mission from self-assembled ZnO microcrystallite thin films,” Appl. Phys. Lett., Jun. 22, 1998, Vol. 72, No. 25, pp. 3270-3272, America Institute of Physics, College Park, Md.] and short wavelength light-emitting diodes (LEDs) [SOOHWAN JANG et al., “Simulation of vertical and lateral ZnO light-emitting diodes,” Journal of Vacuum Science & Technology, pp. 690-694, B 24, 690, American Vacuum Society, New York, N.Y.]. Several researchers have studied p-type ZnO with various dopants [K. MINEGISHI et al., “Growth of p-type zinc oxide films by chemical vapor deposition,” Jpn. J. Appl. Phys., Nov. 1, 1997, Vol. 36, pp. L1453-L1455, Part 2, No. 11A, Japanese Journal of Applied Physics, Tokyo, Japan], [RYU, Y. R. et al., “Synthesis of p-type ZnO films,” J. Cryst. Growth, Vol. 216, pp. 330-334, Elsevier Science B. V., Amsterdam, The Netherlands]. However, the reliability of p-type ZnO is still controversial [LOOK, D. C. et al., “P-type doping and devices based on ZnO,” Phys. Status Solidi, Vol. (b) 241, pp. 624-630, WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim, Germany], and even with successful growth of p-type ZnO, very low hole concentrations [TSUKAZAKI, A. et al., “Repeated temperature modulation epitaxy for p-type doping and light-emitting diode based on ZnO,” Nature Materials, Dec. 19, 2004, Vol. 4, pp. 42-44, Nature Publishing Group, London, United Kingdom] and high resistivity [LOOK, D. C., “Electrical and optical properties of p-type ZnO,” Semicond. Sci. Technol., Mar. 15, 2005, Vol. 20, pp. S55-S61, IOP Publishing Ltd., Bristol, United Kingdom] are still problems for implementing high-performance of ZnO based applications.
Also of interest to this disclosure, Nakayama et al. described that the calculated valence band offset depends on interface configuration of zinc oxide (ZnO)/gallium nitride (GaN), claiming valence band offset values ranging from 1.0 to 2.2 eV [NAKAYAMA, T. et al., “Electronic structures of hexagonal ZnO/GaN interfaces”, J. of Crystal Growth, Jun. 2, 2000, Vol. 214, pp. 299-303, Elsevier Science B. V., Amsterdam, The Netherlands]. Furthermore, VAN DE WALLE, C. G. et al., “Universal alignment of hydrogen levels in semiconductors, insulators and solutions,” Nature, Jun. 5, 2003, Vol. 423, pp. 626-628, Nature Publishing Group, London, United Kingdom, indicated that calculated valence band offset was 1.3 and 1.5 eV for GaN/ZnO and InN/ZnO, respectively.